A cloudy sky that turns into a luminous cover at night. Light pollution, which deprives astronomers of access to the starry sky on clear nights, has an even more severe effect on biodiversity when the sky is overcast. The reflection of light on clouds causes a sharp increase in light pollution. Artificial light levels can increase by a factor of 10 to 30 compared to levels reached under clear-sky conditions. The consequences for the movements and reproduction of nocturnal species are thereby multiplied.

The two false-color images below show the entire sky at 360° from the same observing point, in this case the top of a hill near the small village of Cervières in the Haut-Forez region of France. South is at the center of the image.

• The image on the left is taken on a clear night and, in addition to the stars and the Milky Way, we can see the light pollution halos of the major cities located at distances of between 30 and 85km on the horizon (from left to right Roanne, Lyon, Saint-Etienne and Clermont-Ferrand). The small halo in the center is that of the town of Noirétable located 3.5km away.
  

• In contrast, the image on the right is taken under completely overcast conditions with low clouds. The halos are not at all the same, and light pollution is multiplied by the clouds, which effectively reflect light from nearby sources (light cover effect). From left to right, the halos are those of the village of Cervières (which has not yet switched off its street lighting at 11 p.m.), the Haut-Forez freeway service area and the village of Les Salles (3.5 and 2 km away respectively), with Noirétable in the center. Under these conditions, it is possible to read a book without a flashlight, which is not at all the case under clear sky conditions.
  

Move the cursor to the center of the image to compare the two images obtained under clear and overcast skies.
The images were calibrated and processed with Sky Quality Camera (SQC) software.

The colors used in both images correspond to levels of sky brightness (or luminance) ranging from brightest on the left to darkest on the right. Typical conditions where certain luminance levels are found are indicated on the scale below.

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